As shown in FIG. 1, a prior art elevator comprises an oil pressure mechanism 1 for actuating an oil pressure cylinder 2 to move up and clown. The oil pressure cylinder 2 is fastened at one end thereof with an elevator cab 3. The oil pressure mechanism 1 is provided with a decompression valve 4, which is intended for use by an elevator maintenance personnel to lower the elevator cab 3 in the event of a power outage, an elevator breakdown, or an elevator maintenance work. However, the elevator cab 3 may be caused by a power outage or accident to stop somewhere between two floors such that the door of the elevator cab 3 can not be opened, and that the passengers are trapped in the disabled cab.